Electrical appliances often include mechanisms that terminate operation of the motor in response to thermal overload conditions that could result in permanent damage to the motor or associated equipment. A thermal overload, such as an excessively high winding or rotor temperature, may occur as a result of a locked rotor, a high mechanical load, a supply overvoltage, a high ambient temperature, or some combination of these conditions.
Conventional thermal cut-outs (TCOs) are based on a thermally responsive element that fuses in response to a thermal overload condition, and which thereby interrupts the flow of electrical power to the protected apparatus, One typical approach uses a spring loaded contact pin or lead that is held in electrical connection with an opposing contact by a fusible material such as solder. Another typical approach uses one or more springs, which are independent from the electrical contacts, that drive the contacts apart into a displaceable fusible stop material. These known approaches have several significant drawbacks. In the first approach, the electrical power flows directly through the fusible material. Thus, self-heating in the fusible material itself, particularly in high power applications, can seriously impair the ability of the TCO to be responsive only to the temperature of the protected apparatus. In both described approaches, the TCO typically comprises a complex arrangement of springs and contact elements that are mounted in a housing. Thus, these approaches are costly, and do not allow for the direct inspection of the TCO because the fusible material and contact conditions are not usually visible through the housing.